Ha, and I completely think the opposite. I was busted down to Cpl on a summary trial. Had I opted for the Court Martial (ooh, cringe in fear of those words, NOT) then the powers of punishment would have been far greater (life in prison) BUT the fact that it is a completely JUST process (neutral as in they use the facts and no prior chatting in the mess the night before the trial) would have resulted in a far lesser punishment.
In a CM, the judge is neutral and only looks at fact, the lawyers are neutral and look out for the best interests of their clients and the lawyers don't only fight for the sentencing decision, they debate and decide the punishment as well, using precedences, career history, etc.
Unless the charge is silly and you were caught completely red handed, opt for the CM. (but even then, the lawyers do their jobs and can find technicalities, obscure laws and other examples to help you win).
Just my 2 cents.